1. Introduction: Symptoms of failure
A typical failure of a key joint, in particular the Bosch component 0820024998, often begins with a barely noticeable increase in the level of vibration when the drive shaft is operating. Technical personnel observe backlash in the connection, which is accompanied by a specific metallic knock when changing the direction of rotation or when starting/stopping the equipment. If not diagnosed in time, wear progresses to catastrophic key failure or shaft groove damage, resulting in production line downtime.
2. Overview of the component
Bosch 0820024998 is a standardized torque transmission element (key) conforming to DIN 6885 or ISO 773. standards. Its function is to provide a fixed connection between the shaft and the hub (pulley, gear), transmitting axial and torque. The operating conditions for this component are characterized by high cyclic loads, vibrations and the need for an exact dimensional fit to minimize backlash.
3. Evidence of refusal
During the disassembly of the unit, the following signs were found:
- Fretting corrosion: The presence of small metal particles and traces of oxidation on the faces of the key.
- Shear deformations: Microscopic plastic deformations of the edges of the key, which indicate that the yield point of the material has been exceeded.
- Vibration analysis: Vibration peaks were detected at the shaft rotation frequency exceeding the limit of 7.1 mm/s according to ISO 10816-3.
- Measurements: The gap in the connection exceeds the allowable 0.05 mm for tension fit.
4. Investigation of root causes
The method of the Ishikawa diagram ("fishbone" diagram) was used for system analysis:
| Category | Factors |
|---|---|
| Method | Incorrect tightening torque of hub bolts; ignoring installation procedures. |
| Equipment | Insufficient hardness of the shaft; wear of the groove (key channel). |
| Material | Inconsistency between the hardness of the key and the shaft (the key should be softer, but not too much). |
| Measurements | Errors in the control of tolerances at the stage of production or repair. |
Analysis of the 5 Whys indicates cyclic fatigue failure due to stress concentration in the corners of the groove.
5. Identified root causes (Ranked list)
- Misfit mismatch (50%): Excessive clearance in the joint leads to shock loads when changing the direction of the torque.
- Cyclic overloads (30%): The operation of the equipment beyond the limits of the design loads leads to the exhaustion of the fatigue strength resource of the material.
- Stress concentration (20%): Insufficient rounding radii in the shaft groove create cells for cracks to appear.
6. Corrective actions
Immediate:
- Replacement of the Bosch key 0820024998 with a new one according to the technical conditions.
- Carrying out capillary control (Dye Penetrant Inspection) of the shaft groove to detect hidden cracks.
Long-term:
- Transition to the use of keys with increased strength and precision grinding.
- Implementation of tolerances according to ISO 286 (shaft and hole fit).
- Installation of moment limiters to protect against peak loads.
7. Diagnostic checklist (for technicians)
- [ ] Check for backlash in the connection (visually and mechanically).
- [ ] Measure the width of the groove of the shaft and the hub (in micrometer).
- [ ] Carry out capillary control for cracks (ISO 3452-1).
- [ ] Check the runout of the shaft using a watch-type indicator.
- [ ] Analyze the vibration spectrum (pay attention to frequencies that are multiples of revolutions).
- [ ] Clean surfaces of fretting corrosion products.
- [ ] Check the hardness of the key material (HRC).
- [ ] Check the compliance of the tightening moments of the fastening elements.
- [ ] Visually inspect the corners of the groove for chips.
- [ ] Document the node uptime (MTBF).
8. Prevention strategy
It is recommended to implement a schedule of planned and preventive works (PPR) with an interval of 3000 working hours. Condition Monitoring should include periodic vibration control and node temperature control (threshold 85°C). Updating the design by using locking assemblies instead of classic keys can completely eliminate the problem for highly loaded nodes.
9. Summary
Timely diagnostics and adherence to tolerances during installation are crucial to ensure reliable operation of keyed joints. To purchase certified components and receive technical advice, visit UNITEC-D E-Catalog.
10. Links
- ISO 773:1986. Rectangular keys and their corresponding keyways.
- ISO 10816-3. Mechanical vibration — Evaluation of machine vibration by measurements on non-rotating parts.
- DSTU 2406-94. The pins are prismatic.
- Bosch Rexroth Technical Manuals. Maintenance and Installation Guidelines for Linear and Rotary components.
